This invention relates to the production and treatment of ferrochromium and, in particular, but not exclusively, to the smelting of chromite ore to produce ferrochromium, as well as to the further treatment of ferrochromium fines to a condition in which they are in a more acceptable and pure form.
Insofar as this invention relates to the melting of ferrochromium fines, the only process of concern is the melting of ferrochromium fines, together with solid carbonaceous reductant, in order to achieve improved yields, as well as the melting of fines. Thus the area of melting of ferrochromium fines together with solid carbonaceous reductant, as far as this invention is concerned, could be considered tantamount to smelting in view of the reduction which takes place of unreduced chromite ore often contained in slag portions of ferrochromium fines.
Thus, in broad principle, the invention relates primarily to the smelting of chromite ores in the presence of carbonaceous reductant material in order to produce ferrochromium. Such chromite ores may have undergone some form of pre-treatment such as concentration, pre-heating, pre-oxidation, pre-reduction or pre-leaching. Also, they may be agglomerated, pelletized or briquetted.
Smelting of many different types of chromite ore, whether as a lumpy ore, as briquettes, or as ore fines, in a conventional submerged arc type of furnace, invariably results in appreciable losses of potentially reduceable oxides of iron and chromium to the slag. These losses are largely in the form of unreduced or partly reduced chromium spinel. As a result of this, recoveries of as low as 65% to 70% are often regarded as acceptable.
Smelting in a submerged arc furnace takes place beneath a burden of feed material which automatically feeds into the reaction zone under the influence of gravity. This type of feeding denies any sort of reasonable control over the rate at which feed material is fed into the reaction zone beneath the electrodes. As a result, irrespective of sophisticated computerised control which can be applied to such furnaces, satisfactory recoveries on an absolute scale are not generally achieved.
Even in order to achieve the modest recoveries which are at present regarded as acceptable, selection of suitable carbonaceous reducing agents is necessary and, such reducing agents are very often more costly than other carbonaceous reducing agents, such as coal, which should, technically speaking, be adequate for the purpose.
Applicant believes that in the case of presently used techniques and equipment the liquidus temperature of the slag is very often not fully reached as a result of which the chromite fails to dissolve, and thus be reduced rapidly, as opposed to the relatively extremely slow solid state reduction of chromite. This phenomenon may be attributed to the lack of control over the feed material in a submerged arc furnace.
It is accordingly the object of this invention to provide a process for the production and treatment of ferrochromium wherein the overall recoveries of chromium are substantially improved and, whilst not necessarily being the case, less costly carbonaceous reductants can be employed.
In this specification the term "stoichiometric" is intended to mean the quantity of reductant required to reduce all the oxides of chromium and iron to the metallic or carbide form and to produce the required level of silicon in the product (normally 2 to 4%). Thus the stoichiometric quantity of carbonaceous reductant is calculated on the fixed carbon content of the reductant.
Also, the term transferred arc thermal plasma is defined at least for present purposes, as an electrically generated plasma in which the ion temperature lies in the range 5000 K. to 60,000 K. and the molten material in the bath forms a substantial part of the electrical circuit.